Make Playground URL configurable

[hmn53]

Problem

Hi! Thanks for this great project.

I have been using mdbook for quite some time now. As others have mentioned, sometimes the official playground server is down or slow. The projects I have been using with mdbook requires to run the code a lot of times, which becomes difficult sometimes due to timeout.

Also I have seen others mention that they want to run code for languages other than rust as well.

Proposed Solution

We can make the playground url inside book.js configurable so that the user can point it to some other playground or even to their own server.

Thanks for your time.

[ehuss]

Thanks for the report! I believe this is a duplicate of #350, so closing in favor of that.